Process of continuous distillation

ABSTRACT

Method and apparatus for continuous distillation in which products are removed from the ends of a distillation column and are also removed from an intermediate zone of the column, subjected to a refrigeration cycle and returned at least in part to the column, and reflux and reboiling are provided at more than one zone of the column.

United States Patent 1191 Bligh June 4, 1974 [5 1 PROCESS OF CONTINUOUSDISTILLATION 2,627,731 2/1953 Benedict 62/31 2,677,945 5/1954 Miller62/40 [76] Inventor: Bemafd Ramsay Bhgh, 2,s23,523 2/1958 Eakin 62/26Jarnes 5 Ave., Hampton H111, 2,919,554 1/1960 Kohler 62/34 Mldd e eEngla 3,339,370 9/1967 Streich 62/40 3,592,015 7/1971 Streich 62/40 [22]1971 3,605,423 9/1971 Stoklosinski 62/28 [21] Appl. No.: 133,913

- Primary Examiner-Norman Yudkoff Assistant ExaminerA. Purcell [52] US.Cl 62/40, 62/28, 62/31 51 rm. (:1. F25j 3/02, F25j 3/08, F25j 5/00 Age,Haffne [58] Field of Search 62/23, 24, 27, 28, 29,

62/31, 34, 40, 33 [57] ABSTRACT Method and apparatus for continuousdistillation in [56] References C'ted which products are removed fromthe ends of a distil- UNITED STATES PATENTS lation column and are alsoremoved from an interme- 1,594,336 7/1926 Mewes 62/31 diat n of thcolumn, subjected t a rig rati n 2,146,197 2/1939 Twomey 62/28 cycle andreturned at least in part to the column, and 2,180,435 11/1939 Schlitt62/31 reflux and reboiling are provided at more than one 2,469,7245/1949 Gross 62/28 zone f the column 2,600,110 6/1952 Hachmuth 62/262,608,070 8/1952 Kapitza 62/33 7 Claims, 6 Drawing Figures VAPOR ILIQUID PATENTEDJUH 4197 SHEET 1 0F 4 m MEDQE N mmDmv;

INVENTOR B. R.

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PATENIEDJUH 4:914

sum 2 or 4 PATENTEDJUN 41974 SHEU 3 0F 4 mw mzDOI INVENTOR B. '12. B2L LPROCESS OF CONTINUOUS DISTILLATION This invention relates to the art ofcontinuous fractional distillation, (the term continuous is used hereinto distinguish this process from batch distillation). It is well-knownto those skilled in this art that if a distillation column has refluxadded only at the top of the column, and reboiling applied only at thebottom of the column, the thermodynamic efficiency of the process cannever approach 100 percent. It has been shown that the thermodynamicefficiency of a distillation column can be increased by having one ormore increments of reflux at points between the feed zone and the topreflux zone, and by having one or more increments of heat added betweenthe feed zone and the bottom reboiler, but it has been found that themultiplicity of heaters and coolers make a complexity which outweighsthe advantages of high thermodynamic efficiency.

This problem is particularly significant in distillation processes inwhich the refluxtemperature is below the ambient atmospherictemperature. In these circumstances the process requires a refrigerationsystem. It follows that if the thermodynamic efficiency of thedistillation is to be increased and if conventional refrigeration cyclesare used, a number of refrigeration cycles are required, and thisrequirement adds to the complexity.

An example of this problem is to be found in a process for makingethylene, in which light naphtha is subjected to steam pyrolysis; theproducts of this pyrolysis are hydrogen, methane, ethylene, propyleneand a range of other hydrocarbons. This mixture is separated by lowtemperature distillation, and at least four distillation columns arerequired to produce substantially pure ethylene and substantially purepropylene. This well-known process has a poor thermodynamic efficiencywhich by the prior art can only be improved by a number of refrigerationcycles and a multiplicity of heaters and coolers in some of theaforesaid four distillation columns. There are processes, known to thoseskilled in the arts, by which ethylene is used to manufacturepolyethylene, ethylene glycol and the tere-' phthalate ester of ethyleneglycol; the said ester is polymerized to a substance used for making afibre; propylene is used to manufacture polypropylene and propyleneglycol.

It is the object of the present invention to provide a process forcontinuous distillation which is thermodynamically more efficient than aconventional distillation process (this latter being one in which refluxis added only at the top of a column and reboiling is applied only atthe bottom of the column).

It is a further object of the present invention to provide the saidefficient distillation process in which the coldest zone of thedistillation process is below the ambient atmospheric temperature.

It is a particular object of the invention to provide a process forseparating pyrolysis gas into ethylene and other constituents.

Another object of the invention is to provide a process for separatingnatural gas into a methane-rich fraction and an ethane-rich fraction.

It will be appreciated by those with a wide knowledge of chemicalengineering that this invention will find other applications, forexample if a source of natural gas has appreciable quantities of propanein it this invention can be applied to the process for obtaining asubstantially pure propane fraction.

According to the present invention there is provided a distillationcolumn with a means for withdrawing a stream from a zone between theends of the column, a means for passing the said stream through arefrigeration cycle, a means for returning at least some of the saidstream to the distillation column, a means for providing reflux at morethan one zone of the column and a means for providing reboiling at morethan one zone in the column.

understood by those skilled in the arts and require no furtherdescription herein.

The first form of the invention is described with reference to FIG. 1. Adistillation column I, most suitably constructed with plates 24 anddowncomers 25, has its coldest zone 26 below 5C; line 2 is the feedpipe; line 3 comes from a reservoir of liquid which is fed from adowncomer and which is below line 2. This liquid is a mixture of thecomponents being distilled and is called the refrigerant herein. Part ofthis stream passes through line 4, is cooled in heat exchanger 5, and islet down in pressure at valve 6, to the main reflux condenser 7 wherethe refrigerant vaporizes at least partially. The condensate so createdpasses via vessel 18 to pump 19. Pump 19 delivers the liquid partly tothe top of the distillation column and partly to line 20 as top product.From the main reflux condenser 7'the refrigerant vapor, which maycontain some liquid, passes via line 8 to heat exchanger 5 where it iswarmed and vaporized completely. This refrigerant vapor is then warmednearly to atmospheric temperature in heat exchanger 9 prior to enteringcompressor 10.

A second portion of the liquid refrigerant from line 3 passes throughline 11, is cooled in heat exchanger 12, and is let down in pressure atvalve 13 by such an amount that the refrigerant temperature falls to alevel appropriate for carrying out some condensation at a zone in thedistillation column between line 2 and the top of the column. Therefrigerant vaporizes at least partially in heat exchanger 14 which issuitably a number of tubes or coils erected in the said zone. From heatexchanger 14 the refrigerant vapor, which may contain some liquid,passes via line 15 to heat exchanger 12 where it is warmed and vaporizedcompletely. This refrigerant vapor is then warmed nearly to atmospherictemperature in heat exchanger 9 prior to entering compressor 10.

v The two refrigerant streams are combined in the compressor anddelivered at such a pressure that the vapor condenses in heat exchanger16 which is cooled by water near atmospheric temperature. The liquidrefrigerant is then'cooled in heat exchanger 9 and passes back to thedistillation column via valve 17. The entry point 23 of this refrigerantis just above the reservoir mentioned above. Because some of therefrigerant flashes off at valve 17, this zone of the distillationcolumn acts like an intermediate reboiler.

The heat for the main reboiler 21 may come either from steam or from anyother appropriate source; line 22 takes the bottom product.

In this plant description and in all the descriptions which follow, allheat exchangers are of the indirect type. Heat exchangers such as 5 and'9, which are depicted in FIG. 1 as rectangles, are preferably, but notessentially, of the plate and corrugation type. Heat exchangers depictedas circles, such as 7 and 16, are preferably, but not essentially, ofthe tube in shell type. This convention, which is well-known in the artof chemical engineering, is used in all the figures inthis patentspecification.

There are many minor variants on the first form of the invention. Forexample, if the low pressure gas streams leaving heat exchangers 5, 12,are near to atmospheric temperature then heat exchanger 9 is omitted.

If the distillation column has a number of feed pipes entering atdifferent zones, heat exchanger 14 (herein called the intermediatecondenser must be above at least one of these feed zones, and theintermediate reboiling zone 23 must be below the intermediate condenserand below at least one of the feed zones.

The second form of the invention is described with reference to FIG. 2.A distillation column 31 has its coldest zone 26 below 5C; line 32.isthe feed pipe; line 33 takes refrigerant from a reservoir of liquidwhich is below line 32. The refrigerant is cooled in heat exchangers 34,35, and is let down in pressure at valve 36 to the main reflux condenser37 where the refrigerant vaporizes partially. The refrigerant liquid andvapor are disengaged in vessel 38; the vapor is used to cool heatexchanger 35; the liquid is passed to the intermediate condenser 44where it is vaporized at least partially. The refrigerant from theintermediate condenser 44 rejoins the vapor from vessel 38 at line 46.The combined streams are warmed in heat exchangers 34, 39, and entercompressor 40. After the compressor the refrigerant is condensed in thewater cooled heat exchanger 41 and cooled in heat exchanger 39 beforebeing let down at valve 42 and returned to the distillation column at apoint just above the reservoir.

The main reflux pump is item 47; the top product is withdrawn via line43, and the bottom product via line 45.

This form of the invention has many minor variants comparable with thevariants of the first form. If the refrigerant contains a substantialproportion of a perma-' nent gas such that the mixture cannot becondensed by cooling water in heat exchanger 41, then an auxiliaryrefrigeration system is required; (an example of this case is givenlater in this specification).

If the distillation column has a number of feed pipes entering atdifferent zones, the intermediate condenser 44 must be above at leastone of these feed zones, and the re-entry point at valve 42 must bebelow the intermediate condenser and below at least one of the feedzones.

The third form of the invention is described with reference to FIG. 3. Adistillation column 531 has its coldest zone 26 below 5C; line 532 isthe feed pipe. Via line 533 a mixture (herein called the refrigerant) iswithdrawn from the vapor space in a zone 544, which is above the feedzone and below the top reflux zone.

Line 533 passes to a disengagement vessel 545 where the greater part ofany spray is separated from the vapor; the liquid collects at the bottomof vessel 545 and runs back to the distillation column by gravity vialine 546, which contains a U-bend in order to keep this line primed withliquid. Vapor from vessel 545 passes along pipe 534 to heat exchangers535, 539, where it is warmed successively to atmospheric temperaturenearly.

The vapor enters compressor 540 and after delivery it is cooled in heatexchanger 553 by water near to atmospheric temperature. The highpressure gas is divided in two parts; the first part is cooled in heatexchanger 539 and then condensed in heat exchanger 547, which is anintermediate reboiler in a zone of the distillation column between thefeed zone and the main reboiler 551. The intermediate reboiler issuitably a tube bundle fitted into a downcomer which is sized for thepurpose. The refrigerant leaving heat exchanger 547 is cooled in heatexchanger 535 and then let down in pressure via valve 543 into zone 544;this stream provides intermediate reflux.

The second high pressure stream from heat exchanger 553 is cooledsuccessively in heat exchangers 538, 541, 542, at which point therefrigerantis liquid. The refrigerant is let down in pressure via valve536 to the reflux condenser 537 where it vaporizesat least partially.The reflux so created passes via vessel 548 to pump 549. Pump 549delivers liquid partly to the distillation column top and partly to line550 as top product. From the main reflux condenser 537 the refrigerantvapor, which may contain some liquid, passes through heat exchangers542, 538, where it is warmed nearly to atmospheric temperature beforeentering compressor 540.

The manner in which liquid refrigerant is produced in this cycle dependson the thermodynamic properties of the refrigerant. If it contains asubstantial proportion of a permanent gas, then some auxiliaryrefrigeration is required for heat exchanger 541. Alternatively if therefrigerant at a high pressure is condensable at atmospherictemperature, heat exchanger'54l can be omitted.

If the distillation column has a number of feed pipes entering atdifferent zones, the intermediate reflux zone 544 must be above at leastone feed pipe, and the intermediate reboiler 547 must be below theintermediate reflux zone and below at least one feed zone.

in another variant of this form of the invention there are a number ofintermediate reboilers at different zones of the distillation column,each of these reboilers is subject to the stipulations of the previousparagraph.

in the description which follows temperatures are 9 given in degreescentigrade and pressures in atmospheres absolute where 1.000 atmospheresX 1.013 dynes per square centimetre.

The process is described with reference to FIGS. 4A, 4B and 4C. Feed gasenters the plant at about 1.2 atm and about 25C via line 61 and vessel62. The gas is compressed in a three-stage compressor 63, 64, 65, to apressure most suitably 22.4 atm. The compressor is installed with watercoolers 66, 67, 68, and disengagement vessels 69, 70, 71, which collectorganic and aqueous condensates. The water coolers 66, 67, cool the gasto about 28C and cooler 68 to 26C. The aque ous liquid is rejected vialines 72, 73, 74. The vapor from vessel 71 is passed through tower 79for removal of carbon dioxide by sodium hydroxide solution and throughtower 80 containing water for removing traces of sodium hydroxide. Thevapor is then cooled in heat exchangers 81, 82, and the mixed vapor andcondensate pass to vessel 84. The cooling is effected by the condensatestreams 76, 77; stream '76 is mostly C C and C hydrocarbons and about0.1 percent molar ethylene glycol injected via line 83. The ethyleneglycol prevents the formation of solid hydrates. The mixture is let downto 1.3 atm at valve 85 and it partially vaporizes in heat exchanger 82;this liquid-vapor mixture is returned via line 87 to vessel 62 where theliquid is withdrawn via line 89 and the vapor is recompressed.

Stream 77 is mostly C C, and C hydrocarbons and ethylene glycol isinjected via line 78 to the proportion of about 0.1 percent molar. Themixture is let down to 3.5 atm at valve 86 and it partially vaporizes'inheat exchanger 81; this liquid-vapor mixture is returned via line 88 tovessel 69 where an aqueous liquid is withdrawn via line 72, an organicliquid is withdrawn via line 75, and the vapor is recompressed.

Heat exchangers 81, 82, may suitably be like dephlegmators erected suchthat aqueous liquid can be withdrawn from the bottom. Aqueous liquid isalso withdrawn via line 90 from vessel 84; hydrocarbon condensate iswithdrawn via line 91 to the distillation column 130 to be describedlater. Vapor from vessel 84 passes via line 92 to tower 93 containingalumina for drying. The vapor is then cooled in heat exchangers 94, 96,and the condensate collects in vessel 97 and passes to distillationcolumn 101 via pump 98 and heat exchanger 94. The cooling in heatexchanger 96 is done by the products, hydrogen, methane, ethylene andethane. The vapor from vessel 97 passes to distillation column 100.

The distillation columns 100 and 101 together make up the primaryde-propanizer; the condensers 102, 103, provide the main reflux, whichis returned to the top of column 100 via vessel 104 and pump 99;intermediate reflux is provided by coolers 105, 95; cooler 105 operateswith cooling water at about 22C; heat exchanger 95 is cooled by theproducts. Column 100 operates at 21.0 atm and column 101 at 2 I .2 atm.Pump 106 takes liquid from the bottom of column 100 to the top of column101. Part of the condensate from cooler 105 returns to column 101 vialines 107, which contains a U-bend to maintain a vapor-free stream; theremaining part of this condensate is fed to a zone near the middle ofcolumn 100 by means of pump 148.

The main reboiler 108 is heated by steam; the bottom product, which hasa major proportion of C hydrocarbons and a minor proportion of C is-sentvia line 109 to the secondary de-propanizer 130.

The secondary de-propanizer, which operates at about 10.7 atm, has threefeeds, line 109, line 91, and pump 129 from line 75. Item 131 is a watercooled partial condenser, 132 is a condensate drum, 133 is a refluxpump, 134 is a steam heated reboiler. The top product 137 containsmostly C hydrocarbons with smaller amounts of others such as ethylene;it is returned to compressor stage 65 by way of vessel 70. The bottomproduct 135 contains C and C hydrocarbons, and goes to the de-butanizer136. I

The top product of the primary de-propanizer 100, 101, containshydrogen, light hydrocarbons and about 0.3 percent molar C,; it leavesvessel 104 as vapor at about 20C. It is warmed by heat exchangers 111,112, 113, leading to vessel 114 containing catalyst for the selectivehydrogenation of acetylenes. Heat exchanger 113 is heated by steam. Theprocess gas is cooled again by heat exchangers 112, 115, 111; heatexchanger 115 is water cooled and after it comes vessel 116 for removingcondensable impurities.

Cold process gas leaving heat exchanger 111 passes along line 117 andis'then split into streams 118 and 141. Stream 141 is partially condensedin heat exchanger 119 by cold product streams consisting of hydrogen andmethane. Stream 118 is partially condensed in heat exchangers 120, 121,122, 123, 124, and the process gas and condensate pass to vessel 125which is at about -56C. The gas from vessel 125 is warmed to 42C in heatexchanger 123 and leaves as stream 127. The liquid from vessel 125 iswarmed and partially vaporized in heat exchanger 121 and leaves asstream 128. Heat exchanger 124 is cooled by boiling ethane product at61C; heat exchanger 122 is cooled by boiling ethylene product at -55C,and exchanger by gaseous ethylene at -55 to 22C.

Streams 127, 128, are feed lines to the de-ethanizer, which is operatedaccording to the first form of the invention. The de-ethanizer isconveniently installed as three vessels 200, 201, 202, each containingplates and downcomers; the bottom section of vessel 201 serves as areservoir for liquid. The de-ethanizer operates in the range 19'to 20atm.

From vessel 201 liquid refrigerant at about 3C which is predominantly C5and C hydrocarbons passes along line 207and divides into two streams;part is cooled in heat exchanger 208 and then divides again; onefraction is let through valve 224 to a pressure of about 6.3 atm andcools heat exchanger 225 which is the intermediate condenser for thede-ethanizer; the refrigerant vapor with some liquid passes to heatexchanger 208 where the refrigerant vaporizes completely; this streamcontinues to heat exchanger 215 where it warms up to 22C and enterscompressor 217; another fraction of the cooled refrigerant passes alongline 226 to valve 227 where it is let down to about 6.5 atm for thereflux condenser 103 of the primary depropanizer; the vapor boiling offtogether with some liquid passes along line 228 to heat exchanger 208.

The second main refrigerant stream from line 207 is cooled in heatexchanger 209 and then divides; one fraction is let through valve 246 toa pressure of about 2.1 atm and provides refrigeration successively toheat exchangers 210, 211. The refrigerant boiling off and some liquidpass along line 229 to heat exchanger 209 where the refrigerantvaporizes completely; this stream continues to heat exchanger 215where-it warms up to 22C and enters compressor 216. Another fraction ofthe cooled refrigerant from heat exchanger 209 passes along line 230 tovalve 231 where it is let down to about 2.1 atm for cooling heatexchanger 233, which is part of the refrigeration system for thede-methanizer. The refrigerant boiling from heat exchanger 233 and someliquid pass along line 232 and enter heat exchanger 209.

Refrigerant compression takes place in three stages 216, 217, 218, withwater coolers 219, 220, 221, the final delivery pressure being 38 atmwhich is adequate to bring about condensation in cooler 221 at about28C. The condensate passes into vessel 222 and thence it is cooled inheat exchanger 215 and returned to vessel 201 via valve 223.

The main reboiler 234 of the de-ethanizer is heated by steam, and thebottom product, which is almost entirely propylene and propane, iswithdrawn via line 235, and may be separated by another distillationcolumn (not shown). 1

The overhead vapor from vessel 200 is partially condensed in heatexchanger 211, the condensate collects in vessel 212 and is returned asreflux via pump 214.

Vapor from the top of vessel 201 passes via line 203 to the intermediatereflux condenser 225 and thence the liquid-vapor mixture passes to thebottom of vessel 200; liquid from the bottom of vessel 200 istransferred to the top of vessel 201 via pump 204. Between vessels 201,202, the vapor transfer line is 205, and the liquid transfer line is206. The feed line 127 enters the deethanizer above the first plate fromthe bottom of vessel 200; the feed line 128 enters at about two platesabove the reservoir in vessel 201.

From the de-eth'anizer the top product which contains about 0.4% C,hydrocarbons is withdrawn as vapor from vessel 212 via line 236 at 52C.This stream is partially condensed in heat exchanger 210 and thecondensate at 66C collects in vessel 213, and is withdrawn via line 237as feed to the de-methanizer.

The vapor from vessel 213 passes along line 238 to the cooling trainconsisting of heat exchangers 239, 240, 241, 242, and'thence to vessel243 at about 1 19C. The liquid is withdrawn from vessel 243 via line 248to heat exchanger 240 where it partially vaporizes; this mixture is fedto the de-methanizer via line 249.

The vapor from vessel 243 is cooled to l40C in heat exchanger 244 andpasses to vessel 245; condensate from vessel 245 is fed via line 250 tothe demethanizer top section 300. From vessel 245, vapor which is mostlyhydrogen is warmed to -l25C in heat exchanger 244; the gas is then putinto an expansion engine, must suitably a turbine 255, where the gas iscooled to about -146C at a discharge pressure of 6.2 atm; the dischargegas is put through heat exchanger 244 and then through a secondexpansion engine 254, where the gas is cooled to about l46C. at adischarge pressure of 2.1 atm; this discharge gas makes another passthrough heat exchanger 244, and then it warms up successively throughheat exchangers 239, 119, 96, 95. For the equipment below 1 19C thematerial of construction is metal free from iron and chromium.

The de-methanizer is a variant of the second form of the invention andis conveniently installed as three vessels 300, 301, 302, operating atabout 18.7 atm. Vessel 300 is most suitably a column with plates anddowncomers; and has the main reflux condenser built into the top; feedline 250 enters vessel 300 at a zone about four plates from the top.Vessel 301 has a reservoir at the bottom for the liquid refrigerant,about six fractionating plates in the middle and the intermediate refluxcondenser at the top; vapor passes up the tubes of this condenser andpartial condensation takes place; the boiling refrigerant is in thespace surrounding these tubes; the uncondensed vapor leaving the top ofthese tubes passes to the bottom of vessel 300 via line 303; liquid fromthe bottom of vessel 300 passes to the top plate of vessel 301 via line304 which contains a U- bend to maintain a vapor-free stream. Stream 249is fed onto a plate near the middle of vessel 301. An overflow line 306from the reservoir provides reflux for the bottom vessel 302, whichcontains suitable plates and downcomers; vapor from the top of vessel302 passes up to vessel 301 via line 305.

From vessel 301 liquid refrigerant, which is a mixture of methane,ethylene and ethane at about C, passes along line 322 and is cooled inheat exchangers 310, 311. This stream is let down to 1.5 atm at valve312 into the space round the tubes of the main reflux condenser. Thevapor boiling off at about l44C passes through heat exchangers 311, 310,317, and is warmed to 22C before entering compressor 318, 319. A liquidpurge 313 from the refrigerant of the main reflux condenser is splitinto two streams; stream 314 goes to the intermediate reflux condenserand thence via line 323 and heat exchangers 310, 317, to the compressor;the second purge fraction 315 goes to heat exchanger 241 and thence vialine 316 and heat exchanger 317 to the compressor. The compressordelivers the refrigerant at 21 atm and has water coolers 320, 321. Thehigh pressure gas is cooled in heat exchanger 317, partially condensedin the intermediate reboiler 309, and completely condensed in heatexchanger 233 at about 66C. The condensed refrigerant then goes to thereservoir via line 307 and valve 308. The feed line 237 enters near thetop of vessel 302, and the reboiler 309 is below this feed zone.Reboiler 309 consists of tubes immersed in liquid either in a downcomeror on a plate.

Reboil for the bottom of vessel 302 is provided by withdrawing liquid atline 324, and sending it to the reflux'condenser 102 of the primaryde-propanizer by pump 325 and line 326; the vapor so produced isreturned to the bottom of vessel 302 via line 328.

Line 327 is a branch from line 324 and it takes the mixture of liquidethylene and liquid ethane to the C splitter 400, 401, which operates atabout 15.5 atm. Ethylene vapor at about 37C is withdrawn from the top ofvessel 400 and is warmed in heat exchangers 402, 403, 404, to 22C beforeentering compressor 405. Part of the ethylene is delivered at 23.5 atmand is' cooled in water cooler 406; it is cooled further in heatexchanger 404 and passes via line 408 to intermediate reboiler 409 whereit condenses. The liquid ethylene then takes line 410 to heat exchanger402 where it is cooled; part of this stream is withdrawn as product vialine 416, and part enters the top of vessel 400 as main reflux via valve411.

The second stream from compressor 405 is delivered at 30.9 atm andcooled in water cooler 407; this stream is cooled in heat exchanger 404and condensed in the main reboiler 413. The liquid ethylene is cooled inheat exchanger 403 and enters the top of vessel 400 via valve 415. i

Liquid from the bottom of vessel 400 enters the intermediate reboiler409 and the partially vaporized mixture (424) enters the disengagementvessel 418. Thence the vapor returns to vessel 400 and the liquid runsinto vessel 401 via valve 419; line 425 is a liquid overflow pipebetween vessels4l3 and 400.

The liquid ethylene product 416 is divided into two parts; one part iswithdrawn as liquid via line 417; the other part goes via line 420 tovalve 426, it is let down to 8.9 atm and used to cool the process gas inheat exchangers 122, 120, 96, and used to cool the intermediatecondenser 95.

The liquid ethane product passes along line 422 and is cooled in heatexchanger 126; it is let down at valve 421 to 3.5 atm and it cools theprocess gas in heat exchanger 124; it is then warmed in heat exchangers126, 96, 95.

From the top of the de-methanizer liquid methane is withdrawn via line329; it is let down to 9.4 atm at valve 330 and it is used to cool heatexchangers 242, 239, 119, 96, 95. A gaseous purge stream 331 from thetop of the de-methanizer is mostlymethane with a small proportion ofhydrogen; it is warmed to -7lC in heat exchanger 239 and then putthrough expansion engine 247 most suitably a turbine; the delivery gas332 is at about l4lC and 2 atm and is used to cool heat exchangers 239,119, 96, 95.

The aqueous liquid residues from vessels 62, 69, may be treated for therecovery of ethylene glycol, for example by distillation.

lt is to be understood that in the example given the conditions oftemperature and pressure are open to variation depending on thecomposition of the feed gas and on the temperature of the cooling wateravailable; alternatively, air coolers can be used.

TABLE 1 Typical composition of gas to he separated Mole The three basicforms of the invention are shown in F168. 1, 2 and 3 with the topproduct delivered as liquid; there are some variants in which the mainreflux condenser is a partial condenser and the top product is deliveredpartly or entirely as a gas; examples of such variants respectively arethe de-methanizer in FIG. 4C and the de-ethanizer in FIG. 48.

I claim:

1. A process for the continuous fractional distillation of a fluidcontaining two or more substances at least one of which has a boilingpoint below ambient temperature in which the main liquid reflux isprovided by condensing at least part of the vapors leaving the top ofthe distillation column and in which the main reboiling is provided at azone near the bottom of the distillation column, characterized in that:

a. a refrigeration cycle is used to provide the main reflux at thecoldest zone of the column by indirect heat exchange and the refrigerantused in the refrigeration cycle is a fluidwithdrawn from an intermediatezone of the column;

b. the refrigeration cycle includes the stages of compression,condensation, expansion and evaporation, and at least part of therefrigerant is returned to a zone of the column near to the intermediatezone of withdrawal; I

c. in addition to the main reflux condenser at the top of the column andthe main reboiler at the bottom of the column, there is at least oneother zone at an intermediate level in which intermediate reflux isapplied and at least one other zone at an intermediate level in whichintermediate reboil is applied;

d. the refrigeration cycle, which provides the main reflux by indirectheat exchange, is common with the cycle which provides intermediatereflux and intermediate reboil.

2. A process as claimed in claim 1 wherein a stream of liquid iswithdrawn from an intermediate zone below the zone into which the fluidbeing fractionated is fed and is divided into two further streams one ofwhich, after cooling, is passed through a condenser in the columnbetween the said feed zone and the top of the column causingcondensation of vapor and refluxing of the resulting liquid down thecolumn, and the other of which after cooling passes to the main refluxcondenser at the top of the column where the condensed liquid formed isreturned in part at least to the top of the column as reflux, and bothstreams after passing through the reflux condensers are warmed near toambient temperature, are subjected to compression, condensation andfurther cooling and returned via an expansion valve to the column at azone just above that from which the original stream leaves, theflash-gas at said expansion valve providing intermediate reboiling.

3. A process as claimed in claim 1 wherein a stream of liquid iswithdrawn from an intermediate zone below the zone into which the fluidbeing fractionated is fed, and after cooling is passed to the mainreflux condenser which provides reflux to the top of the column, thesaid stream boils partially in the said main reflux condenser and thevapor and liquid paits of the said stream are separated, the liquid partpassing to one or more reflux condensers between the top of thedistillation column and the feed zone, the liquid part vaporizing atleast partially and then combining with the above vapor part, therecombined streams are warmed near to ambient temperature, are subjectedto compression, condensation and further cooling and returned via anexpansion valve to the column at a zone just above that from which theoriginal stream leaves, the flash-gas at the said expansion valveproviding intermediate reboiling.

4. A process as claimed in claim 1 wherein a stream and top reflux areobtained, after which this part of the stream is warmed near to ambienttemperature, compressed and joined with the other compressed vapors, thecombined stream being cooled and divided into two parts as mentionedabove, one part of which goes through a heat exchanger in the column andre-enters the column near the zone from which the original stream waswithdrawn.

5. A process according to claim 1 in which there is more than oneprocess fluid introduced by way of more than one feed stream to thefractional distillation column for the purpose of fractionation.

6. A process as claimed in claim 1 wherein the fluid being fractionatedis derived from pyrolysis gas and the separated products are mainlyhydrogen, methane, ethylene, ethane and propylene. v

7. An apparatus for carrying out the process of the continuousfractional distillation of a fluid containing one or more substances, atleast one of which has a boiling point below ambient temperature, inwhich the main liquid reflux is provided by condensing at least part ofthe vapors leaving the top of the distillation column and in which themain reboiling is provided at a zone near the bottom of the distillationcolumn, characterized in that, said apparatus comprises I a. means forproviding at one or more intermediate zones intermediate reflux andmeans for providing at one or more other intermediate zones intermediatereboil in addition to the said main reflux and main reboiling;

b. means for withdrawing a stream of fluid from an intermediate zone ofthe column;

0. means for putting said stream through a refrigeration cycle includingthe stages of compression, condensation, expansion and evaporation, thesaid cycle providing means for refrigeration by indirect heat exchangeat the main reflux condenser and providing means for the saidintermediate reflux and intermediate reboil; and

d. means for returning at least part of said stream to the column nearto said intermediate zone of withdrawal.

2. A process as claimed in claim 1 wherein a stream of liquid iswithdrawn from an intermediate zone below the zone into which the fluidbeing fractionated is fed and is divided into two further streams one ofwhich, after cooling, is passed through a condenser in the columnbetween the said feed zone and the top of the column causingcondensation of vapor and refluxing of the resulting liquid down thecolumn, and the other of which after cooling passes to the main refluxcondenser at the top of the column where the condensed liquid formed isreturned in part at least to the top of the column as reflux, and bothstreams after passing through the reflux condensers are warmed near toambient temperature, are subjected to compression, condensation andfurther cooling and returned via an expansion valve to the column at azone just above that from which the original stream leaves, theflash-gas at said expansion valve providing intermediate reboiling.
 3. Aprocess as claimed in claim 1 wherein a stream of liquid is withdrawnfrom an intermediate zone below the zone into which the fluid beingfractionated is fed, and after cooling is passed to the main refluxcondenser which provides reflux to the top of the column, the saidstream boils partially in the said main reflux condenser and the vaporand liquid parts of the said stream are separated, the liquid partpassing to one or more reflux condensers between the top of thedistillation column and the feed zone, the liquid part vaporizing atleast partially and then combining with the above vapor part, therecombined streams are warmed near to ambient temperature, are subjectedto compression, condensation and further cooling and returned via anexpansion valve to the column at a zone just above that from which theoriginal stream leaves, the flash-gas at the said expansion valveproviding intermediate reboiling.
 4. A process as claimed in claim 1wherein a stream of mainly vapor is withdrawn from a zone between thefeed zone and the top of the column and is then warmed near to ambienttemperature, compressed, cooled and divided into two parts, one of whichis condensed in a heat exchanger in the column below the feed zone inorder to provide intermediate reboiling while the other part is cooledand condensed and goes to the main reflux condenser from which topproduct and top reflux are obtained, after which this part of the streamis warmed near to ambient temperature, compressed and joined with theother compressed vapors, the combined stream being cooled and dividedinto two parts as mentioned above, one part of which goes through a heatexchanger in the column and re-enters the column near the zone fromwhich the original stream was withdrawn.
 5. A process according to claim1 in which there is more than one process fluid introduced by way ofmore than one feed stream to the fractional distillation column for thepurpose of fractionation.
 6. A process as claimed in claim 1 wherein thefluid being fractionated is derived from pyrolysis gas and the separatedproducts are mainly hydrogen, methane, ethylene, ethane and propylene.7. An apparatus for carrying out the process of the continuousfractional distillation of a fluid containing one or more substances, atleast one of which has a boiling point below ambient temperature, inwhich the main liquid reflux is provided by condensing at least part ofthe vapors leaving the top of the distillation column and in which themain reboiling is provided at a zone near the bottom of the distillationcolumn, characterized in that, said apparatus comprises a. means forproviding at one or more intermediate zones intermediate reflux andmeans for providing at one or more other intermediate zones intermediatereboil in addition to the said main reflux and main reboiling; b. meansfor withdrawing a stream of fluid from an intermediate zone of thecolumn; c. means for putting said stream through a refrigeration cycleincluding the stages of compression, condensation, expansion andevaporation, the said cycle providing means for refrigeration byindirect heat exchange at the main reflux condenser and providing meansfor the said intermediate reflux and intermediate reboil; and d. meansfor returning at least part of said stream to the column near to saidintermediate zone of withdrawal.